1. Field
The present invention relates to a deionization apparatus, and more particularly to a capacitive deionization apparatus that removes ion components inside a fluid (liquid or gas) using an electrochemical method.
2. Description of the Related Art
Water, especially groundwater, contains a great amount of minerals such as calcium and magnesium. A total amount of such calcium or magnesium that has been expressed numerically is referred to as hardness in water. In this regard, hard water has high numbers and soft water has relatively small numbers.
When using hard water, i.e. water with high hardness, for a washing machine or a dish washer, the water reacts with a detergent causing a problem of degrading detergence. When using hard water for a steam generation unit or a heating unit, the water forms scales causing problems of reducing energy efficiency and nozzle clogging.
That is, hard water in home appliances, which use water, degrades detergence and accumulates a great amount of scales in the channel where water flows. Thus, the reliability of the product can be deteriorated.
In order to solve these problems, a water softener using an ion exchange resin has been suggested.
The water softener using an ion exchange resin softens water by exchanging hard water components Ca2+ and Mg2+ contained in water with NaCl and Na+ injected in the ion exchange resin. This water softener using an ion exchange resin is inconvenient, because NaCl must be injected to the ion exchange resin periodically, and the ion exchange resin itself must be replaced due to impurities contained in water. Moreover, the method of using ion exchange resin has disadvantages for being uneconomical, because acidic or basic solutions must be used in recycling the resin and a great amount of polymer resins and chemicals must be used to treat a great amount of water.
Recently, active researches on capacitive deionization (hereinafter, abbreviated as ‘CDI’) apparatus to overcome the above-mentioned disadvantages have been conducted.
As shown in FIG. 1, a CDI technology is based on a simple principle that removes ions dissolved in a fluid such as water by applying a voltage between two porous electrodes such that anions are adsorbed to a positive electrode and cations are adsorbed to a negative electrode. In the CDI technology, the electrodes are easily recycled, because the ions adsorbed to the electrodes can be separated (desorbed) by oppositely changing the polarity of the electrodes or terminating the power supply when the ion adsorption to the electrodes is saturated. The CDI technology does not use detergent solutions such as acid or base to recycle the electrode as in the methods using an ion exchange resin or reverse osmosis. Thus, there are advantages in that no secondary chemical wastes are generated and the lifespan of the electrodes is semi-permanent due to a little corrosion and contamination.
FIGS. 2A and 2B are views illustrating a unit cell structure and a power connection of a conventional CDI apparatus.
As shown in FIG. 2A, upper part of a unit cell 20 (A region in FIG. 2A) of a conventional CDI apparatus is connected to an anode of a power supply 30 and consists of a current collector 12 to apply power to electrodes 14 and a positive electrode 14. Lower part of the unit cell 20 consists of a current collector 12 to connect with a cathode of the power supply 30 and a negative electrode 14. Further, the current collector 12 and electrodes 14 are adhered to each other using a conductive binder (conductive double-sided tape 18) between the current collector 12 and the electrodes 14. The water softening process is carried out by having a fluid (liquid or gas) pass between the upper and lower part of the unit cell 20 and adsorbing ion components inside the fluid to the electrodes 14.
The unit cell 20 of the conventional CDI apparatus is connected to the power supply 30 by fastening a bolt to a bolt fastener hole 12a formed on the current collector 12.
Ideally, when connecting the unit cell 20 to the power supply 30, a supply voltage (Vp) of the power supply 30 should all be applied between the positive electrode 14 and the negative electrode 14 (Vp=Vc). However, in practical, a contact impedance caused by the bolt fastening and a parasitic impedance result in a voltage loss (Vloss) as shown in FIG. 2B.
That is to say, only a voltage which excluded the voltage loss (Vloss) from the supply voltage (Vp) of the power supply 30 is applied between the positive electrode 14 and negative electrode 14. At this time, the voltage loss (Vloss) is calculated by the following equations 1 and 2.Voltage Loss (Vloss)=Current flowing in circuit of unit cell (Ip)×Total impedance inside unit cell (Ztotal)  [Equation 1]Total impedance inside unit cell (Ztotal)=Contact impedance caused by bolt fastening (Zbolt)+Parasitic impedance caused by current collector (Zcurrent collector)+Parasitic impedance caused by conductive binder (Zbinder)+Parasitic impedance caused by electrodes (Zelectrodes)  [Equation 2]
Particularly, among the total impedance inside the unit cell 20 (Ztotal), the factor which gives biggest influence on the voltage loss is the contact impedance caused by the bolt fastening. The contact impedance is greatly influenced by a shape of the bolt and nut fastened to the bolt fastener hole 12a of the current collector 12 and a degree of tightening of the bolt and nut. That is, when the conventional CDI apparatus is used for a prolonged time, the degree of tightening becomes loosened, and as a result, the contact impedance is increased.
FIG. 3 is a view showing a stack structure of a conventional CDI apparatus. A base unit structure that forms the stack structure of the conventional CDI apparatus is shown in B region of FIG. 3. The base unit structure consists of a current collector 12 to apply power to electrodes 14, and a pair of electrodes 14 each arranged above or below the current collector 12. The unit structure may further use a conductive binder (conductive double-sided tape 18) between the current collector 12 and the electrodes 14 to adhere the current collector 12 and the electrodes 14 together (however, the conductive binder is not necessarily used, because the impedance of the conductive binder tend to reduce electrical efficiency of a system).
The conventional CDI apparatus described with reference to FIGS. 2 and 3 has the following problems.
First, the base unit structure that constitute the stack structure of the CDI apparatus, that is, the current collector and electrodes are not integrally bonded. Thus, there is a problem that products with defects are manufactured during the assembly of the stack structure. Especially, graphite, which is commonly used for the current collector 12, has a nature to be easily broken, that usually results in production defects during the process to form the stack structure.
Secondly, the contact impedance generated by the bolt fastening between the current collector 12 and the power supply 30 causes insufficient voltage applied between the positive electrode 14 and the negative electrode 14. Thus, there is a problem that deionization efficiency is reduced, and as a result, the efficiency of a system is deteriorated.
Thirdly, the stack structure of the conventional CDI apparatus has many power connection terminals (bolt fastening holes) to apply power to each unit cell 20. Thus, there is a problem that the power application is difficult.